Typical conventional photoreceptors for electrophotography contain one of the following photosensitive layers on an electrically conductive support: a vapor-deposited layer of Se, Se-Te alloy or Se-As alloy, a layer having CdS or ZnO particles dispersed in a resin, a layer having the particles of phthalocyanine pigment derivative dispersed in a resin, and an organic photoconductive layer containing poly-N-vinylcarbazole (hereinafter referred to as PVK) and/or 2,4,7-trinitro-9-fluorenone (hereinafter TNF). When these photoreceptors are subjected to cyclic use, the photosensitive layer is fatigued or its surface is mechanically damaged or worn. The causes of the fatigue are bombardment due to corona discharge during the charging step, and the degradation due either to active gases produced during charging or to light illumination. The causes of mechanical damage or wear are the friction with a receiving sheet as it is peeled after the toner image has been transferred to it, and the contact with the material to clean the drum. For these reasons, it becomes necessary quite soon to mirror-polish the surface of the photosensitive layer, or replace the photoreceptors using other photosensitive layers.
It is known to form a protective layer on the photosensitive layer to prevent its fatigue or surface wear or damage. Japanese Patent Publication No. 24414/77 discloses a photoreceptor for electrophotography, including a photoconductive photosensitive layer of selenium, selenium-arsenic or selenium-tellurium that is coated with a protective layer of a thickness of 0.5 to 5.mu., that comprises a combination of three resins selected from among polyester, polyurethane, polyvinylidene chloride, acetylcellulose, polyacrylonitrile and polyvinyl chloride. Japanese Patent Application (OPI) No. 3749/78 (the symbol OPI as used hereunder means an unexamined published Japanese patent application) discloses a photoreceptor for electrophotography including a photoconductive photosensitive layer of selenium, selenium-tellurium, selenium-arsenic or selenium-tellurium-arsenic that is coated with a protective layer of a thickness of 0.2 to 5.mu. that comprises a combination of glycol-modified silicone resin and one resin selected from among an acylic resin, acryl-urethane resin, urethane resin, polyester resin and epoxy resin.
These two photoreceptors are adapted for use in cyclic transfer electrophotography using the Carlson process, but the protective layer in the first type of photoreceptor is not capable of the desired protection against fatigue, particularly against mechanical wear if its thickness is only a few microns, and a significant improvement is achieved only if the thickness is increased to several tens of microns. But if such a thick protective layer is formed on the photosensitive layer, not all the electrical charges can be removed from the photosensitive layer by either optical or electrical erase means, and as the photoreceptor is subjected to cyclic transfer electrophotography, the buildup of residual charge causes considerable fog and makes the subsequent formation of an image impossible. In spite of this fact, Japanese Patent Publication No. 23910/67 proposes a photoreceptor that uses a very thick resin layer to protect a photosensitive layer. This photoreceptor has an insulating resin layer of a thickness of about 100.mu. formed on a photosensitive layer having photoconductive CdS particles dispersed in a resin binder, and it claims the prevention of not only fatigued photosensitive layer due to light, active gases or electron bombardment but also mechanically damaged photosensitive layer. But on the other hand, the use of this photoreceptor has led to the need of the complex image forming process described in that patent, namely, the "NP process" that consists of charging, imagewise exposure, reverse charging (or a.c. charging) and full-frame exposure and which requires two different charging devices. This inevitably results in a complex image forming apparatus and process.
Another method to protect the photosensitive layer is to use a double-layer photoreceptor comprising a charge transport layer and a charge generating layer, and specific examples of this method are described in Japanese Patent Applications (OPI) Nos. 116930/79, 1943/81 and 60446/81. In each patent, the photoreceptor comprises a fairly thin photoconductive layer of high sensitivity and low resistance that includes a photosensitive layer which generates carriers upon illumination with visible light. Generally, such charge generating layer is used as the underlying layer and is formed on a substrate, and on that layer is formed a charge transport layer that is capable of transporting the charges generated in said charge generating layer. This transport layer is made of a relatively thick, transparent photoconductive layer that has high resistance and good physical properties, and is generally made of a photoconductive resin layer or a resin layer containing a photoconductive material. When the above named patents were filed, the inventors expected that the photosensitive layer (charge generating layer) would be protected from light, electron bombardment and active gases, as well as from mechanical wear, damege and moisture and that it would provide a photoreceptor adapated for use in the Carlson process with minimum fatigue. But as it turned out, the charge transport layer that comprised resins which were inherently low in mechanical properties could not be subjected to long-term cyclic operation without mechanical damage or loss in the ability of transporting charges. Japanese Patent Publication No. 25218/74 proposed a three-layer photoreceptor that comprised an underlying highly photosensitive photoconductive layer, a thin intermediate layer of insulating organic compound and an overlaying photoconductive layer that absorbs light in a different wavelength region than that of the photosensitive layer. The underlying layer is typically made of arsenic triselenide and has high sensitivity and low resistance, and this is overlaid with a thin insulating layer made of an inorganic compound such as SiO.sub.2, ZnS or MgF.sub.2, and this intermediate layer is further coated with a photoconductive resin layer typycally comprising polyvinyl carbazole that absorbs light only in the ultraviolet or short wavelength range. In this three-layer arrangement, the intermediate layer and overlaying layer protect the underlying photosensitive layer from fatigue and wear, but because of its three-layer structure, two different charging devices, i.e. a d.c. corona discharger and an a.c. corona discharger, are necessary as demonstrated below, and this again results in a complex image forming apparatus and process. When the surface of this photoreceptor is subjected to a uniform negative charging, positive charges are induced in the vicinity of the intermediate layer since the photosensitive layer of arsenic triselenide has low resistance (this completes the first step). Then, the surface of the photoreceptor is subjected to a.c. corona discharge, and at the same time, it is subjected to imagewise exposure using light to which only the underlying layer is sensitive, and an electrostatic latent image is formed (this completes the second step). Subsequently, to increase the contrast of the latent image, the photoreceptor is subjected to full-frame exposure with light that is absorbed by the overlaying layer (this completes the third step). The so treated photoreceptor is subjected to the conventional transfer method comprising development of the toner image, transfer of the developed image to a receiving sheet, typically paper, and fixing the transfer image.
As described above, no photoreceptor for cyclic transfer electrophotography has been found that can be subjected to long-term cyclic operation without buildup of residual charges that cause fog and fatigue, and no image forming method that uses such photoreceptor has been found either.